Broadband Spectral Energy Distributions of Active Galactic Nuclei from an Accretion Disk with Advective Coronal Flow

We construct a disk-corona model which accounts for the optical-to-X-rayspectral properties of Seyfert nuclei and QSOs. We study emission spectrumemerging from a vertical disk-corona structure composed of two-temperatureplasma by solving hydrostatic equilibrium and radiative transferself-consistently. A fraction $f$ of viscous heating is assumed to bedissipated in a corona with a Thomson optical depth of $\tau_c$, whereadvective cooling is also included, and a remaining fraction, $1-f$, dissipateswithin a main body of the disk. Our model can nicely reproduce the soft X-rayexcess with a power-law shape and the hard tail extending to $\sim$ 50 keV. Thedifferent spectral slopes ($\alpha \sim$ 1.5 below 2keV and $\sim$ 0.5 above,where $\Fnu \propto \nu^{- \alpha}$) are the results of different emissionmechanisms and different sites; the former slope is due to unsaturatedComptonization from the innermost zone and the latter is due to a combinationof the Comptonization, bremsstrahlung and a reflection of the coronal radiationat the disk-corona boundary from the inner to surrounding zone ($\leq$ 300Schwarzschild radii). The emergent optical spectrum is redder ($\alpha \sim0.3$) than that of the standard disk ($\alpha \sim -0.3$), being consistentwith observations, due to the different efficiencies of spectral distortion ofdisk emission at different radii. Further, we find that the cut-off frequencyof the hard X-ray ($\sim$ coronal electron temperature) and broad-band spectralshape are insensitive to the black-hole mass, while the peak frequency of thebig blue bump is sensitive to the mass as the peak frequency $\propto\Mbh^{-1/4}$.Comment: 12 pages, 9 figures, accepted for publication in the Astrophysical Journa